1. Field of the Invention
This invention relates generally to liquid crystal display devices, and more particularly to a method and apparatus for preventing unwanted reduction of contrast in images created thereby. A predominant current usage of the inventive improved optical system with angular compensation is in the correction of angularly dependent error caused by birefringence in reflective liquid crystal display devices, particularly but not exclusively in off axis optical systems, wherein angular dependant differences in the index of refraction will tend to create unintended changes in the brightness of an image.
2. Description of the Background Art
In a liquid crystal display (LCD”) imaging apparatus, it is important to carefully regulate the amount of light which forms each pixel of a displayed image, so as to provide the correct amount of contrast between lighter and darker portions of an image.
FIG. 1. is a block diagrammatic representation showing a typical reflective display based three color projection system 100, illustrating the operation of a polarizing electro-optical imaging system. The projection system 100 has an illumination source 102, a polarizing beam splitter 104, a color separator 106, a plurality of reflective liquid crystal displays (LCDs) 108(r, g, and b), and projection optics 110. Illumination source 102 generates a source beam of white light and directs the source beam toward polarizing beam splitter 104, which passes one portion of the source beam having a first polarity, and redirects another portion (an illumination beam) of the source beam having a second polarity along a system axis 112, toward color separator 106. Color separator 106 separates the illumination beam into its red, green, and blue components, and directs each of these colored illumination beams to a respective one of LCDs 108(r, g, and b). Each of LCDs 108(r, g, and b) is controlled by a system, e.g., a computer or other video signal source (not shown), and modulates the polarity of selective portions (i.e., pixels) of the colored illumination beams to form colored imaging beams, which are reflected back toward color separator 106. Color separator 106 recombines the colored imaging beams to form a composite imaging beam and directs the composite imaging beam back along system axis 112, toward polarizing beam splitter 104, which passes only the modulated portions of the composite imaging beam to projection optics 110. Projection optics 110 then focuses the modulated portions of the composite imaging beam onto a display surface (not shown).
The example of FIG. 1 is an “on-axis” system, in that the beams from the beam splitter 104 to the color separator 106 and also the beams from the color separator 106 back through the beam splitter 104 are on the common system axis 112. Even in such an example, some rays of the beam will impinge upon the LCDs 108 at different angles as compared to other rays, although the chief, or average, ray will be essentially perpendicular to the surface of the LCDs 108. As can be appreciated by one skilled in the art, and as will be discussed in more detail hereinafter, in “off-axis” systems, wherein the two paths diverge, the chief, or average, ray will be far from perpendicular to the surface of the LCDs 108.
Typically, in a reflective liquid crystal display apparatus such as the three color projection system 100, each portion of light (light ray) which is to represent a pixel in a resultant image is polarized with a polarizer (such as the polarizing beam splitter 104 of FIG. 1), and is then directed through a liquid crystal material such as is found in the LCD's 108, and then exits toward the projection optics 110 through an “analyzer”. In the “on axis” example of FIG. 1, the beam splitter 104 serves both as the polarizer and as the analyzer.
There are two problems associated with the angular dependence of the retardation of the LC material. Note that any angular variation of the retardation will change the way in which the LCD modifies the polarization of light, and therefore will change the system performance, almost always in a negative fashion. In most cases, the LC is designed to work optimally on axis, that is, for a ray of light that impinges upon the LCD imager perpendicular to its surface. In any real display system, the light hitting any given point on the projection screen will come from a whole range of angles. The farther these rays are from the perpendicular at the LCD, the further the operation of the LCD for these rays will be from optimal. While the details of the angular distribution will depend on the details of the optical design, there will be a significant range of angles for all such designs. In general, this variance from perpendicularity will degrade the performance of the system, most noticeably by lowering the contrast.
For the on axis design of FIG. 1, the center ray of this range of angles will be close to perpendicular at the LCD, but even in this case, there will be a degradation of performance because of the many non-perpendicular rays. For the off-axis design of FIG. 2, however, the center ray will in general be far from perpendicular, and it is possible that none of the rays will be so.
Secondly, such angular change will be different in different areas of the liquid crystal display apparatus. As discussed above, the light from each pixel on the LCD, which is transferred to a corresponding point on the projection screen, is an average over many angles. For some optical designs, and particularly for the off-axis design of FIG. 2, this set of angles will be different for each and every pixel. Therefore, the undesired contrast shift discussed above will not be uniform across the entire liquid crystal display.
It would be advantageous to have some method or means for preventing unwanted polarization aberrations across the surface of a liquid crystal imaging apparatus.